Only a few weeks after starting university, her son phoned to say he'd joined a Bible group with the London Church of Christ. When he announced he was going on one of their rallies, Rosemary decided to find out more about the church.
"I looked up the church in a Christian journal but it wasn't listed. When William told me he was going to be baptised, alarm bells started ringing."
So she contacted the Cult Information Centre. "After speaking with them, my worst fears were confirmed. Images of Moonies and brainwashing whizzing round my head."
As the months passed, William phoned home less and less. "Eventually, I spoke to the parents of a boy who had also joined a sect. They hadn't heard from him for two years. The father was like a man bereaved."
Rosemary knew the best way to reach William was to get him to return home to Bridgwater, Somerset for the summer. But he said he was staying in London to strengthen his faith.
"I insisted he was coming home but when I went to pick him up, he wasn't the same boy. He was very defensive whenever he mentioned the church, and he was very withdrawn. My son was slipping away and I had no idea what to do to get him back."
A friend said she was going on a holiday organised by a local Christian group. "I'd nothing to lose by sending William on it, too," says the 49-year-old mum. "When he got back, he told me he'd become friendly with a girl there and it made him realise there were good people outside his sect."
The final hurdle came two weeks later when he went on a cult retreat. Rosemary says: "I was terrified it would start again but he rang after two days and asked to be collected. He hasn't been near the cult since."
William, now 25, says: "I thought the church would be a good place to make new friends. Soon I was going to four or five meetings a week. I really believed everyone outside the Church of Christ was misguided.
"It wasn't until the holiday that I saw the cult for what it was. I gave 10 per cent of my student grant to them. I'm so grateful Mum saw the danger."
A spokesman for the Cult Information Centre says: "We're concerned about The London Church of Christ. We have a tremendous number of phone calls from anxious and worried parents about this group."